The present invention relates to a chemical vapor deposition process and apparatus, and to a product made with the process and apparatus which comprises a fiber preform structure which carries a ceramic matrix.
In recent years, ceramics have been the focus of considerable attention for use in advanced energy conversion systems such as heat exchangers, gas turbines and other heat engines. The low fracture toughness of conventional ceramic is a severe limitation to their use in a number of applications.
Considerable work has been carried out over the past twenty years in the development of fiber-reinforced ceramics in an attempt to improve ceramic toughness. It has become known that the toughness of some ceramics can be increased appreciably by reinforcement with high strength fibers. However, conventional ceramic fabrication procedures which generally involve pressing and heating at high temperatures, can lead to damage of reinforcing fibers and result in significant reduction in fiber strength. In order to overcome such problems, chemical vapor deposition (CVD) processes have been developed whereby fibrous forms are subjected to infiltration by reactant gaseous materials which, under controlled conditions, react to form a ceramic matrix on and around the fibers. Thereby, a fiber-toughened ceramic composite is provided without any damage to the fibers.
There are shortcomings in such chemical vapor deposition processes, however. Some of the chemical vapor deposition processes use a flow of gaseous material around a heated fibrous form and depend on diffusion to transport the reactant gases into the fibrous form.
U.S. Pat. No. 4,580,524 to Lackey, Jr., et al, which is incorporated here by reference, comprehensively discloses prior art which is relevant to the present application. This reference teaches of a chemical vapor deposition (CVD) process for preparing fiber-reinforced ceramic composites. It provides for a steep thermal gradient across the thickness of a fibrous preform. Even though it teaches of the preparation of a fiber-reinforced tubular composite, it has been found that a satisfactory tubular composite cannot be prepared. One of the primary problems appears to reside in obtaining the desired temperature of the inner surface of the fibrous preform. When the inner surface of the fibrous preform is not sufficiently cooled, deposition occurs at that surface preventing further flow of reactant gases into the preform.
U.S. Pat. Nos. 4,275,095 and 4,397,901 to Warren disclose a process for encasing fibers with a pyrolytic carbon coating while increasing the surface fracture energy of the composite structure. The composite structure results in a greater toughness and flaw resistance.
An article "The Process Development and Mechanical Testing of a Carbon/Carbon Composite Fabricated by Chemical Vapor Infiltration of a Filament--Wound Substrate" by J. D. Theis, Jr. presented at the 3rd International Conference on Chemical Vapor Deposition in Salt Lake City in 1972 discusses the effect of filament-wound carbon composites at angles of loading greater than or equal to 20.degree. but less than or equal to 45.degree., and at a 70.degree. angle.